Cytostatic or cytotoxic compounds are widely used in the treatment of cancer. Doxorubicin is an aminoglycosidic anthracycline antibiotic and will be used as a typical representative of this group of compounds.
The cell membrane represents a physical barrier and there are some factors that determine the rate of uptake of doxorubicin. The main factors are hydrophobicity (an increase will increase the rate of uptake) and protonation degree of amino group--pKa (a decrease will increase the rate of entry). The doxorubicin inhibits cell growth and has a marked effect on the nuclear material, which becomes non-specifically thickened, agglutinated or broken. The major binding force between doxorubicin and DNA is intercalation of the planar chromophore, stabilised by an external electrostatic binding of the positive charged amino sugar residue with negative phosphate group of DNA.
The intercalated drug molecules appear to prevent the changes in conformation of the helix, which are necessary as a preliminary to initiation of nucleic acid synthesis. The major lethal effect of doxorubicin is inhibition of nucleic acid synthesis. As consequence the drug is more active against dividing cells and the greatest effect is in the S stage of the cell cycle (Brown J. R., Adriamycin and related anthracycline antibiotics in: Progress in Medicinal Chemistry edited by G. P. Ellis and G. B. West, Elsevier/North-Holland Biomedical Press v.15, pp. 125-164, 1978).
Some observations are consistent with the formation complex of electrostatic nature between the positive amino group of doxorubicin and negative phosphate group of phospholipids such as cardiolipin, phosphatidyl serine, phosphatidyl inositol and phosphatidic acid. Cardiolipin is an almost characteristic component of the inner membrane of mitochondria, which are abundant in the cardiac muscle. The pathogenesis of the mitochondrial lesions is one of the major and more specific sub-cellular changes characterizing doxorubicin cardiotoxicity. The rather selective toxicity doxorubicin for mitochondria may be due to the high concentrations of cardiolipin in the mitochondria of the cardiac muscle (Duarte-Karim M., et al. Biochem. Biophys. Res. Comm., v.71, N.2, pp. 658-663, 1976).
The interaction between doxorubicin and lipids has been studied using large unilamellar vesicles (LUVET) composed of mixtures of anionic phospholipids and various zwitterionic phospholipids. Dilution of anionic lipids with zwitterionic lipids leads to decreased membrane association of the drug because electrostatic forces are very important in doxorubicin-membrane interaction. However, binding of doxorubicin to LUVET composed of anionic phospholipids combined with phosphatidylethanolamine (PE) is much higher than binding to LUVET made of anionic lipids plus a range of other zwitterionic lipids such as phosphatidylcholine and the N-methyethanolamine and N,N-dimethylethanolamine derivatives of PE (Speelmans G, et al., Biochemistry, v.36, N.28, pp. 8657-8662, 1997).
The interaction of adriamycin with human erythrocytes was investigated in order to determine the membrane binding sites and the resultant structural perturbation. Electron microscopy revealed that red blood cells incubated with the therapeutic concentration of the drug in human plasma changed their discoid shape to both stomatocytes and echinocytes. The drug was incubated with molecular models. One of them consisted of dimyristoylphosphatidylcholine and dimyristoylphosphatidylethanolamine multilayers, representatives of phospholipid classes located in the outer and inner leaflets of the erythrocyte membrane, respectively. X-ray diffraction showed that adriamycin interaction perturbed the polar head and acyl chain regions of both lipids. It is concluded that adriamycin incorporates into both erythrocyte leaflets affecting its membrane structure (Suwalsky M., Z Naturforsch [C] v.54, N3-4, pp. 271-277, 1999).
The different physicochemical properties of dipalmitoylphosphatidylcholine liposomes with soybean-derived sterols have been studied. Liposomal doxorubicin increased the pharmacological effect compared with free drug, suggesting a decrease of side effect and long circulation (Maitani Y., Yakugaku Zasshi, v.116, N.12, pp. 901-910, 1996).
Liposomes containing polyethylene glycol-derivatised phospholipids are able to evade the reticulo-endothelial system and thereby remain in circulation for prolonged periods. The doxorubicin encapsulated in these sterically stabilised liposomes suppresses the growth of established human lung tumour xenografts in severe combined immunodeficient mice and inhibits the spontaneous metastases of these tumours (Sakakibara T., et al., Cancer Res., v.56, N. 16, pp. 3743-3746, 1996).
A liposome encapsulation can protect surrounding tissue from the cytotoxic effects of the drugs after subcutaneous (s.c.) administration. Liposomes composed of "fluid-state" phospholipids only delayed the damaging effects of doxorubicin when injected s.c. Liposomes with a more rigid nature were much more effective in preventing local tissue damage over a longer period of time when administered s.c. (Oussoren C., et al., Biochim. Biophys. Acta, v.1369, N.1, pp. 159-172, 1998).
Exogenous polyunsaturated fatty acids modulate the cytotoxic activity of anti-cancer drugs in the human breast cancer cell line MDA-MB-231. Among all polyunsaturated fatty acids tested, docosahexaenoic acid was the most potent in increasing doxorubicin cytotoxicity (E. Germain, et al., Int. J. Cancer, v.75, pp. 578-583, 1998).
There remains a need for novel compounds and methods for the treatment of cancer. The present invention aims i.a. to increase the pharmacological activity of presently used anti-cancer drugs, such as doxorubicin, and to introduce novel approaches to the treatment of cancer.